In X-ray generation devices, a DC-to-DC converter may be used for transforming a low DC input voltage (for example 400 V into a high DC output voltage (up to 160 kV). For example, such a DC-to-DC converter may comprise a controllable inverter for generating an AC voltage with variable frequency, a transformer for generating a high AC voltage, and a rectifier for generating the high DC output voltage, which may supply an X-ray tube.
Power inverters for X-ray generation frequently drive resonant load circuits. In particular, medical diagnostic applications may need to control the output power to the X-ray tube in an extremely wide range from quasi no-load to maximum power. One or more switching events of the power semiconductor switches of the inverter shall be controlled accordingly in any half cycle of the resonant current oscillation.
For example, WO 2006/114719 A1 shows DC-to-DC converter with a full-bridge inverter connected to a resonant circuit. Three control modes are defined, which are used for controlling the output power.
However, especially for very low tube output power (e. g. no-load, closed grid of the X-ray tube) the controller may generate a scattering effect which means strongly dithering amplitudes of the resonant current. The result may be a significant low-frequency ripple on the high voltage, which is undesired.